Carburization and oxidation resistant alloy

ABSTRACT

CARBURIZATION AND OXIDATION RESISTANT, AUSTENITIC, IRONNICKEL-CHROMIUM-SILICON ALLOY CONSISTING ESSENTIALLY OF ABOUT: 35-74% NICKEL, 2.5-11.5% CHROMIUM, 1-5.5% SILICON, BALANCE SUBSTANTIALLY ALL IRON. THE PREFERRED COMPOSITION IS ABOUT: 69-71% NICKEL, 2.5-7% CHROMIUM, 2.5-4.5% SILICON, BALANCE SUBSTANTIALLY ALL IRON.

UniteclStafes' Patent O 3,764,304 CARBURIZATION AND OXIDATION RESISTANT ALLOY Alvin E. Nehrenberg and Gene R. Rundell, Lockport, N.Y., assignors to Wallace-Murray Corporation, New York, N.Y.

No Drawing. Continuation-impart of application Ser. No.

740,796, May 21, 1968, which is a continuation-inpart of application Ser. No. 488,321, Sept. 20, 1965, both now abandoned. This application Oct. 9, 1970, Ser. No. 79,645

Int. Cl. C22c 39/20 US. Cl. 75128 C 2 Claims ABSTRACT OF THE DISCLOSURE carburization and oxidation resistant, austenitic, ironnickel-chromium-silicon alloy consisting essentially of about: 35-74% nickel, 2.5-l1.5% chromium, l5.5% silicon, balance substantially all iron. The preferred composition is about: 69-71% nickel, 2.5-7% chromium, 2.5-4.5 silicon, balance substantially all iron.

CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of our copending application Ser. No. 740,769, filed May 21, 1968, now abandoned, which is in turn a continuation-in-part of our application Ser. No. 488,321, filed Sept. 20, 1965, and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to austenitic alloys generally thought of as stainless steels and more specifically ironnickel-chromium-silicon alloys.

The alloys of the present invention fall in that class of alloys which are heat-resisting and which resist deterioration in a variety of industrial atmospheres.

Due to their peculiar chemical and physical characteristics, the alloys of the present invention find use in many situations and in a variety of industrial atmospheres such as are encountered in bright, annealing, hardening, carburizing, nitriding, etc.

The alloys of the present invention may be used in those situations where high strength at elevated temperatures is required, where resistance to oxidation and sealing is a pre-requisite to use, as well as resistance to carburization, sulfur attack, nitriding and other undesired and deteriorating chemical and physical degradation.

More specifically, such equipment and accessories as furnace mufiies, brazing retorts, carburizing boxes, containers for molten lead or salts for heat treating, jet engine and stationary turbine components, resistance heating elements, etc., may be made from alloys of the present invention.

Heretofore, alloys such as Inconel, Hastelloy X, and American Iron and Steel Institute Types 309, 310, 311, 314, 330, etc., the compositions of which are listed below in Table 1, all were stated to depend to a great extent upon the presence of sufiicient chromium in the composition to form a protective oxide film. It was stated that this film serves to prevent deterioration of the underlying metal from internal oxidation, carburization, nitriding, sulfidation, etc. When sufiicient oxygen is pres ent in the environment, the protective oxide film is maintained continuously and the alloy gives very satisfactory service. However, when there is an oxygen deficiency and a source of carbon is present in the atmosphere, the prior art alloys may suffer marked and rapid deterioration from a particular type of corrosion which has been termed green rot. This deterioration occurs at temperatures in the neighborhood of 1800 F. when both carburizing and oxidizing conditions are present simultane- 3,764,304 Patented Oct. 9, 1973 ously or intermittently. Green rot is characterized by the presence of a green deposit on the surface of the alloy or upon a fractured portion of a failed part thereof. Under such conditions, absorption and diffusion of carbon, it is believed, leads to the formation of chromium carbides, the chromium content of the matrix is lowered and the overall oxidation resistance is decreased. When chromium carbides are precipitated as more or less continuous grain boundary networks, the grain boundaries are rendered susceptible to rapid attack by oxidation.

All so-called nickel-chromium alloys heretofore known are susceptible to the type of deterioration just described. Obviously, if carburization could be prevented so that there would be no networks of chromium carbide to provide paths for rapid and deep oxide penetration, much longer service life would be expected. This carburizationoxidation sequence has been a serious problem for many years in the nickel-chromium alloy field, and the solution of it has not been evident to those skilled in the art.

In an article by E. N. Skinner, J. F. Mason and I. J. Moran, High Temperature Corrosion in Refinery and Petrochemical Service, Corrosion, vol. 16, No. 12, December 1960, pp. 593t600t, it is reported that field experience and laboratory examination of field exposed test pieces indicate carburization resistance depends importantly upon chromium, and that its effectiveness is enhanced at higher nickel contents. The article reports an example wherein the amount of carbon absorbed is much less for Type 310 (21% Ni-24% Cr) than for Type 309 (13% Ni-24% Cr) or 304 (9% Ni-19% Cr). These authorities also indicate that increasing silicon at these chromium levels provides increased carburization resistance.

In accordance with the principles of the present invention, these conclusions may be valid for the levels of nickel and chromium there studied, but may not be extrapolated to other levels of composition. As the result of a comprehensive study leading to this invention, it has been found that in high nickel alloys on the order of 35 to 74% nickel, the resistance to carburization increases with decreasing, rather than increasing, chromium. Thus contrary to expectations, we find the carburization resistance of low chromium high nickel alloys to be markedly superior to that of alloys high in chromium. In fact, alloys of this invention are virtually immune to carburization as will become apparent from data which will be presented subsequently.

High nickel-low chromium alloys, per se, have been found unsuitable for service at elevated temperatures in carburizing environments because they are not oxidation or scale-resistant. Consequently, in order for such alloys to be useful, their oxidation resistance must be improved by the addition of one or more of the elements such as silicon, aluminum, rare earths, etc., in sufficient quantity to enhance this characteristic without adversely affecting corrosion resistance.

BRIEF SUMMARY OF THE INVENTION The novelty of this invention resides in the discovery that low chromium-high nickel alloys are virtually immune to carburization and that their low oxidation or scaling resistance can be overcome by critically proportioning chromium, silicon and nickel. The relationship between chromium and silicon is not straight forward. At low chromium levels there is no evidence silicon affects carburization resistance, whereas at intermediate chromium levels, i.e., around 8%, increasing silicon accentuates rather than inhibits, carburization as the prior art suggests. By employing the relationships discovered in our experimental work, it is possible to maintain oxidation resistance in our low chromium carburization resistant alloys equivalent to the oxidation resistance of the higher chromium prior art alloys at temperatures as high as 2200 F. The optimum alloy balance varies with temperature in a manner which will be described later. In accordance with the present invention, alloys containing 35 to 74% nickel, predetermined amounts of silicon from about 1% up to about 5.5% and predetermined amounts of chromium from about 2.5 to 11.5%, may be balanced in such a way that they are more resistant to carburization than prior art alloys. Also, that the adverse eifect on oxidation resistance resulting from the lowering of chromium may be compensated for by properly proportioning the nickel, chromium and silicon contents. When these elements are present in the proportions dictated by a relationship hereinafter defined and discussed, the oxidation resistance of the carburization resistant, low chromium alloys can be made to be equal to that of the best of the prior art alloys at temperatures in the range 1800 to 2200 F.

Some of the nickel may be replaced by manganese without adversely affecting carburization and oxidation resistance. Also, carbon in amounts up to about 0.60 may be added for strengthening or to provide greater fluidity for casting without adversely affecting carburization or oxidation resistance.

In like manner, tungsten or molybdenum may be added to the alloy of the invention in order to obtain greater strength at high temperature as can titanium or columbium. Such additions when made are in the range 1% to 5%. These, however, are special situations, the invention residing in the proportioning of nickel, chromium and silicon, all as described herein. The tensile strength at room temperature of a typical alloy of the present invention is in the neighborhood of 90,000 psi. with a yield of 30,000 psi.

DETAILED DESCRIPTION OF THE INVENTION In Table 1 the composition of typical alloys of this invention are compared with those for commonly used prior art alloys. Note the our alloys contain much less chromium than that used in prior art alloys:

TABLE 1.COMPOSITIONS OF EXPERIMENTAL AND PRIOR ART ALLOYS '(AISI) Carburization tests were performed on specimens which were .444 in diameter by .44" long. They were surface ground to produce a uniform surface finish and were degreased in methanol. The specimens were packed in a commercial carburizing compound and heated to 2000 F. for 100 hours.

One specimen, Hastelloy X, is more carburization re sistant than other prior art alloys, yet the microstructure thereof contains a large volume of carbides resulting from carburization during the 100 hour exposure at 2000 F. in the pack carburization compound. A comparison specimen of our 70% nickel-2.8% chromium-3.5% silicon alloy, on the other hand, which was exposed along with Hastelloy X at the same time in the same carburizing box, completely resisted carburization.

The microstructure of our 70% nickel-4.3% chromium- 3.5 silicon alloy, after it had also been exposed to the carburizing conditions, which so severely carburized Hastelloy X, showed some slight evidence of carburization indicating that the 4.3% chromium alloy is slightly more susceptible to carburization than the 2.8% chromium alloy.

Further evidence supporting our finding that carburization resistance decreases with increasing chromium is summarized in Table 2. Note that the 70% nickel-2.8% chromium alloys with 1.5 to 3.5% silicon show no metallographic evidence of carburization. The almost negligible amount of carburization designated as a trace of carburization in Table 2, is typical for the 70% nickel-4.3% chromium alloy with 2.5% silicon, as well as for the 3.5% silicon alloy used.

When as much as 7.7% chromium is present, our alloys become very definitely susceptible to some degree of carburization and the carburization increases with increasing silicon. This adverse effect of silicon for high chromium contents is an unexpected discovery, since the authorities cited previously, state silicon in amounts up to about 2.5 percent is most effective in preventing carbon absorption, especially in the 18/8 and 25/20 base compositions where such modifications are known as Type 302B and 314, respectively.

TABLE 2.CASE DEP'IHS RESULTING FROM HO UR PACK CARBURIZING AT 2000 F.

Case Alloy depth, designation Ni C-r Si inches C R 22 2. 8 1. 5 None. CR 25".- 2.8 2.5 Do. CR 28-- 2.8 3.5 Do. 0 R 31 4. 3 2. 5 Trace CR 32-. 4.3 3.5 Do. OR 1 7. 7 1. 5 .003. OR 2- 7.7 2.5 .028. OR 3 70 7.7 3.5 .252. Hastelloy X- 48 22. 0 0. 9 .087. Inconel 75 15. 0 0. 2 .130. 309 14 23. 0 0. 7 .154.

In the alloys of this invention we limit the chromium content to about 11.5% maximum, for we have found alloys containing more than this amount of chromium are as susceptible to carburization as alloys which have heretofore been commercially available. The lowest possible chromium content is preferred, for this provides maximum carburization resistance. However, in order to provide sufficient oxidation resistance at temperatures above about 1800 F., more chromium may be required at some sacrifice in carburization resistance. No single alloy composition is optimum since both carburization resistance and oxidation resistance cannot be maximized for all types of commercial carburizing environments and for all ranges of temperature. It will become apparent, as the result of the discussion which follows, that relationships we have discovered during the course of our studies enable us to arrive at the best alloy balance for a particular set of environmental conditions.

Oxidation resistance of the alloys was determined for temperatures in the range 18002200 F. by employing the following procedure. The specimen size was .44" round by .444" long. The specimens were surface ground to produce a uniform surface finish, degreased in methanol and Weighed. They were placed on ceramic trays and were heated in the muffie of a Delaware gas furnace through which air was passed at a slightly positive pressure, the flow rate being about 13 cubic feet per hour. The air was first passed through water to saturate it with moisture to accelerate oxidation.

Both cyclic and isothermal exposures were carried out. In the cyclic tests the specimens were given 16-hour heatings at temperature and were air cooled to room temperature between heating cycles. The loss of weight after an electrolytic descaling operation in a molten mixture 60% NaOH and 40% Na CO at 1050 to 1100 F. was taken as a measure of the extent of metal loss by scaling. The weight loss was converted to a scaling rate expressed in terms of milligrams of Weight loss per square decimeter of surface area per day.

The oxidation rates plotted against the nickel level for several alloys of the invention and show that there is an optimum nickel level for best oxidation resistance in our high nickel, chromium-silicon alloys. This nickel level is about 70%. Useful alloys may be obtained at other nickel levels but this requires that a larger amount of silicon and/ or chromium be added to maintain the desired amount of oxidation resistance. Nickel contents as low as 35% and as high as 74% will yield satisfactory carburization and oxidation resistant alloys when the chromium and silicon are balanced according to the teachings of this invention.

As mentioned previously, optimum alloy balance varies with temperature as well as nickel content and we have discovered that this may be expressed mathematically as follows:

(1) 5.7(percent Cr)=+14.2(percent Si)=K+2(percent Ni70) for percent Ni not greater than 70%, and (2) 5.7(percent Cr)+14.2(percent Si)'=K+2(70percent Ni) for percent Ni not less than 70%.

The value of K for 1800 F. maximum service is 70-75; it is 75-80 for 2000 F. maximum service; and it is 90-95 for 2200" F. maximum service. These values of K minimize the alloy content required for a particular temperature level but it will be understood that excess of the alloying constituents may be used, although the cost may be higher. In any event, the minimum K values of 70, 75 and 90 for maximum service temperature of 1800 F., 2000 F. and 2200 F. respectively should be observed for most successful results.

In practicing the teachings of this invention, and to obtain the best results therefrom, the procedure is as follows: First, a chromium level is selected which depends upon the inverse relationship between carburization resistance and chromium content and upon the degree of carburization resistance required in the particular service environment for which the material is intended. For mildly carburizing conditions the chromium content may be higher than for more severe conditions. Then, enough silicon is added to satisfy the above mathematical relationship so as to assure adequate oxidation resistance.

Experience with our alloys has indicated that the carburization resistance deteriorates when the silicon content is appreciably greater than that required for the desired minimum level of oxidation resistance. For that reason, we specify a maximum value of K as well as a minimum value of K for each level of service temperature. For example, when 7.7% chromium is present with 70% nickel, 2.1% silicon is the minimum required to assure oxidation resistance at 2000 F equivalent to that for the best of the prior art alloys. The corresponding value of K equals 75. Carburization occurs with increasing the silicon in this alloy, but does not become excessive until the silicon exceeds 2.5%, corresponding to a value of K equal to 79.4.

It has been shown that alloys of this invention are vastly superior in carburization resistance to prior art alloys because of their low chromium content, and that their low oxidation resistance which results from the lowering of chromium may be raised to the level of that for the best prior art alloys by properly proportioning chromium and silicon, leaving a net gain in carburization resistance.

In summary, the alloy composition contains nickel, chromium, silicon and iron, wherein the nickel is in the range 35% to 74% chromium is present in a predetermined amount from 2.5% to 11.5% and silicon is present from about 1 to 5.5%, the remainder being substantially iron.

Of the four alloys Which are more generally employed, nickel will be present generally in the neighborhood of and within a preferred range of 69% to 71%, with chromium preferably about 2.5 to 7%, and silicon about 1% up to 5.5%, and generally in the range 2.5 to 4.5%.

What is claimed is:

1. An austenitic alloy consisting essentially of about: 35 to 74% nickel, 2.5 to 11.5% chromium, 1 to 5.5% silicon, up to about 5% each of tungsten and molybdenum, balance iron, characterized by high resistance to carburization and oxidation at temperatures up to about 18002200 F., wherein the nickel is present in an amount not greater than 70% and the chromium and silicon are present in the amounts in percent by weight as follows:

5.7(percent Cr)+14.2(percent Si) =K+2(70percent Ni) where K has a value in the range of 70-95.

2. An austenitic alloy consisting essentially of about: 35 to 74% nickel, 2.5 to 11.5% chromium, 1 to 5.5% silicon, up to about 5% each of tungsten and molybdenum, balance iron, characterized by high resistance to carburization and oxidation at temperatures up to about 1800-2200 F., wherein the nickel is present in an amount not less than 70% by weight and the chromium and silicon are present in the amounts in percent by weight as follows:

5.7 (percent Cr) +14.2(percent Si) -=K|-2(70percent Ni) where K has a value ranging between 70-95.

References Cited UNITED STATES PATENTS 1,528,478 3/1925 Hadfield 75-128 W 1,420,708 6/19-22 Johnson 75-128 R 1,667,746 5/1928 Smith 75128 C 2,750,283 6/1956 Loveless 75128 C 3,366,473 1/1968 Nehrenberg 75128 R 3,385,739 5/1968 Danis 75128 R 'I-IYLAND BIZOT, Primary Examiner US. Cl. X.R. 

